BAT AUDITORY SENSITIVITY SERIES

It is likely that multiple brain circuits process and represent distance information within echoes to form the basis for these distance-based percepts and behaviors. These active adjustments in the sonar signal that occur as the bat closes in on its prey create an optimal signal for localizing the position of the prey item in three dimensions ( Jones and Holdereid, 2007 Moss and Schnitzler, 1995 Simmons and Stein, 1980). As the bat begins to track and approach potential prey, it increases the repetition rate, decreases the duration, and progressively modifies the amount of frequency modulation (FM) in its sonar signals ( Griffin, 1986 Jones and Holdereid, 2007 Kalko and Schnitzler, 1993 Schnitzler and Kalko, 1998 Simmons et al., 1979 Surlykke and Moss, 2000). For example, as an insectivorous bat searches for prey, it emits a sonar signal with long duration and narrow bandwidth that is well suited for target detection. Bats also use this information to adaptively adjust motor control of flight, body position, and sonar vocalizations ( Chiu et al., 2009 Moss and Surlykke, 2001, 2010). A key piece of information obtained from echoes is the distance to a target, used by bats to create percepts that allow them to judge the distance and relative velocity of single targets ( Simmons 1971, 1973 Simmons et al., 1979 Wenstrup and Suthers, 1984) and to distinguish and track multiple elements of a complex environment ( Moss and Surlykke, 2010 Surlykke et al., 2009). In the case of aerial-hawking insectivorous bats, information obtained from echoes is also used to capture flying prey. The extraordinary ability of bats to analyze echoes from self-generated sounds allows them to orient through complex environments in complete darkness. These pathways may contribute to rapid adjustments in flight, body position, and sonar vocalizations that occur as a bat closes in on a target. FM-FM neurons in IC also project strongly to pre-motor centers including the pretectum and the pontine nuclei. Pathways through auditory thalamus undergo radical reorganization to create highly ordered maps of pulse-echo delay in auditory cortex, likely contributing to perceptual features of target distance analysis. A second major role of the midbrain is to project target distance information to many parts of the brain.

BAT AUDITORY SENSITIVITY SERIES

One role of the IC is to create these “delay-tuned”, “FM-FM” response properties through a series of spectro-temporal integrative interactions.

IC neurons are selective for certain delays between frequency modulated (FM) elements of the pulse and echo.

The mustached bat's auditory midbrain (inferior colliculus, IC) is crucial to the analysis of pulse-echo delay. The primary distance cue is the delay between the bat's emitted echolocation pulse and the return of an echo. This regulation of ascending acoustic information by commands originating from higher brain centers may provide the bat with a mechanism to actively control acoustic signal processing and thus optimize acoustic signal analysis.Using their biological sonar, bats estimate distance to avoid obstacles and capture moving prey. The effect of this corticofugal regulation of auditory sensitivity in inferior collicular neurons can also be produced by ionophoretical application of GABA to the collicular recording site. Regulation of auditory sensitivity of recorded inferior collicular neurons was observed when the corticocollicular pathway was activated by electrical stimulation in the auditory cortex. We demonstrate for the first time that the corticocollicular pathway continuously regulates acoustic signal processing in the inferior colliculus by increasing the threshold, reducing the auditory spatial response area, and sharpening the frequency tuning curve of recorded inferior collicular neurons. Using bats as a model auditory system, we studied corticofugal control of auditory sensitivity of neurons in the inferior colliculus.

Bat auditory sensitivity